1. Field of the Invention
The present invention is directed to a method of etching anodic foil for it use in the manufacture of electrolytic capacitors and more particularly, to a method of non-uniform etching of anode foil to produce higher capacitance foil without sacrificing foil strength and to an electrolytic capacitor incorporating the etched anode foil of the present invention for use in an implantable cardioverter defibrillator (ICD).
2. Related Art
Compact, high voltage capacitors are utilized as energy storage reservoirs in many applications, including implantable medical devices. These capacitors are required to have a high energy density since it is desirable to minimize the overall size of the implanted device. This is particularly true of an Implantable Cardioverter Defibrillator (ICD), also referred to as an implantable defibrillator, since the high voltage capacitors used to deliver the defibrillation pulse can occupy as much as one third of the ICD volume.
Implantable Cardioverter Defibrillators, such as those disclosed in U.S. Pat. No. 5,131,388, incorporated herein by reference, typically use two electrolytic capacitors in series to achieve the desired high voltage for shock delivery. For example, an implantable cardioverter defibrillator may utilize two 350 to 400 volt electrolytic capacitors in series to achieve a voltage of 700 to 800 volts.
Electrolytic capacitors are used in ICDs because they have the most nearly ideal properties in terms of size, reliability and ability to withstand relatively high voltage. Conventionally, such electrolytic capacitors include an etched aluminum foil anode, an aluminum foil or film cathode, and an interposed kraft paper or fabric gauze separator impregnated with a solvent-based liquid electrolyte. While aluminum is the preferred metal for the anode plates, other metals such as tantalum, magnesium, titanium, niobium, zirconium and zinc may be used. A typical solvent-based liquid electrolyte may be a mixture of a weak acid and a salt of a weak acid, preferably a salt of the weak acid employed, in a polyhydroxy alcohol solvent. The electrolytic or ion-producing component of the electrolyte is the salt that is dissolved in the solvent. The entire laminate is rolled up into the form of a substantially cylindrical body, or wound roll, that is held together with adhesive tape and is encased, with the aid of suitable insulation, in an aluminum tube or canister. Connections to the anode and the cathode are made via tabs. Alternative flat constructions for aluminum electrolytic capacitors are also known, comprising a planar, layered, stack structure of electrode materials with separators interposed therebetween, such as those disclosed in the above-mentioned U.S. Pat. No. 5,131,388.
In ICDs, as in other applications where space is a critical design element, it is desirable to use capacitors with the greatest possible capacitance per unit volume. Since the capacitance of an electrolytic capacitor increases with the surface area of its electrodes, increasing the surface area of the anode foil results in increased capacitance per unit volume of the electrolytic capacitor. By electrolytically etching an anode foil, an enlargement of a surface area of the foil will occur. Electrolytic capacitors which are manufactured with such etched foils can obtain a given capacity with a smaller volume than an electrolytic capacitor which utilizes a foil with an unetched surface.
In a conventional electrolytic etching process, surface area of the foil is increased by electrochemically removing portions of the foil to create etch tunnels. For example, U.S. Pat. Nos. 4,474,657, 4,518,471 and 4,525,249 to Arora disclose the etching of aluminum electrolytic capacitor foil by passing the foil through an electrolyte bath. The preferred bath contains 3% hydrochloric acid and 1% aluminum as aluminum chloride. The etching is carried out under a direct current (DC) and at a temperature of 75xc2x0 C. U.S. Pat. No. 4,474,657 is limited to the above single step. U.S. Pat. No. 4,518,471 adds a second step where the etched foil is treated in a similar bath with a lower current density and at a temperature of 80-82.5xc2x0 C. U.S. Pat. No. 4,525,249 adds a different second step, where the etched foil is treated in a bath of 8% nitric acid and 2.6% aluminum as a nitrate, at a temperature of 85xc2x0 C.
The ideal etching structure is a pure tunnel-like etch with defined and uniform tunnel diameters and without any undesirable pitting of the foil. As tunnel density (i.e., the number of tunnels per square centimeter) is increased, a corresponding enlargement of the overall surface area will occur. Larger surface area results in higher overall capacitance. However, as tunnel density increases more of the aluminum foil is removed, reducing the strength of the remaining foil. Therefore a compromise must be made between foil strength and capacitance gain.
Traditionally, electrolytic capacitor foil is etched uniformly over the surface. With a uniform, random tunnel etch, the useable capacitance gain of the anode foil is limited by the strength requirements of the foil in its particular application. Thus, there is a need in the art for an etch process which increases the overall capacitance of the foil while retaining foil strength.
To combat this problem, it is suggested in U.S. Pat. No. 5,660,737 to Elias et al. (xe2x80x9cthe Elias patentxe2x80x9d) selectively etch capacitor foil, such that areas which are not subject to stress during manufacturing are highly etched and those areas which are subject to stress during manufacturing are lightly etched or not etched at all. It is suggested in the Elias patent that for a stacked capacitor, the area where the weld tab is to be attached and the periphery of the plate should be masked. Similarly, the Elias patent suggests that for flattened or oval capacitor, in which a wound roll capacitor element is flattened in a press such that the material has a bend at each side and flat areas in between, that high gain foil to be used in the flat areas, while masking strengthens the locations where the sharp bends occur.
Alternatively, International published Application WO 00/19470 to O""Phelan et al. (xe2x80x9cthe O""Phelan referencexe2x80x9d) discloses a foil structure combining the durability of core-etched foils with the electrolyte flow advantages of tunnel-etched foils, having one or more holes or perforations and one or more cavities with a depth less than the foil thickness. The O""Phelan reference discloses that this xe2x80x9cperforated-core-etchedxe2x80x9d foil can be made either by initially core-etching the foil to form cavities and then perforating the core-etched foil, or by initially perforating the foil and then etching the perforated foil to form the cavities. The O""Phelan reference suggests that the perforations can be formed using lasers, chemical etchants, or mechanical dies.
The present invention is directed to a method of etching anode foil in a non-uniform manner which increases the overall capacitance of the foil while retaining foil strength. In effect the foil may be etched to a higher degree in select regions, leaving a web of more lightly etched foil to retain strength.
In particular, by using a mask to protect a mesh grid of the foil from further etching, a previously etched foil can be further etched, prior to the widening step. Alternatively, the mask may be used in the initial etch, eliminating the need for the second process. The higher surface area in the exposed areas does not significantly decrease the strength of the foil as a whole.
In the preferred embodiment, anode foil is initially etched to produce an enlargement of surface area of at least 20 times in a high temperature etch electrolyte. Next, the foil is placed between two masks with a grid of openings which expose the foil in these areas to the etching solution. The exposed area can be as little as 10% of the total foil to as much as 95% of the total foil, preferably 30% to 70% of the total foil area. The pattern is configured in such a way that the enhanced area does not create large scale strength defects such as perforation holes and is held in a pattern such as a hexagonal array, random array or radial burst array, such that the exposed area perimeter can be round, square, hexagonal, or triangular. The masked foil is then replaced into the etching electrolyte solution to further etch the exposed areas of the foil. After widening and forming, a foil etched according to the present invention is suitable for use as an anode in an electrolytic capacitor. The anode foil will have enhanced capacitance without the increased brittleness, which would render the foil unuseable, typical of anode foils highly etch according to conventional methods.